Exploring the long-term temporal variability in polarization through multi-epoch optical spectro-polarimetry - Part I: A sample of Herbig Ae/Be and classical Be stars

Abstract

Polarization signatures across emission line features, together with their temporal evolution, offer a powerful probe of the circumstellar environments of astrophysical sources, on spatial scales otherwise inaccessible to direct imaging techniques. However, the photon-hungry nature of spectro-polarimetry has significantly limited the availability of such datasets in the literature. This work reports a multi-epoch spectro-polarimetric monitoring campaign targeting a sample of Herbig Ae/Be and classical Be stars. The initial observations were obtained during the commissioning and performance-verification phase of ProtoPol, a recently developed medium-resolution echelle spectro-polarimeter mounted on the Physical Research Laboratory (PRL) 2.5m telescope, Mt Abu, India. Given the limited number of comparable datasets available for this class of objects, follow-up observations of the same targets were carried out repeatedly over more than 28 months (December 2023-March 2026), enabling an investigation of the temporal behavior of their polarimetric properties. Our sample comprises 11 Herbig Ae/Be stars and 10 classical Be stars. Our observations found that, while the H$α$ polarization remained relatively constant for the classical Be stars, they showed significant variability for most of the Herbig stars in the sample. The observations presented here constitute one of the rare multi-epoch spectro-polarimetric datasets spanning more than two years and should be of considerable interest to the broader astronomical community. This paper is Part-I of a two-part series of sample studies; corresponding results for symbiotic and red giant stars are presented in Part-II.